Vertically-oriented, lithographically-ordered, metal nanowire arrays (MNAs) have potential utility as capacitors, high surface area electrodes, electrochemical biosensors, optical nanoscopes, rectennas, and solar cells. MNAs are desired for their high specific surface area, high aspect ratio, unity dimensionality, high conductivity, orthogonality to an underlying substrate, and feature sizes on the order of a wavelength of light. MNAs are often coated with materials to produce core-shell or nanocoax structures, which, for many deposition processes, requires the nanowires to be freestanding to accomplish a homogeneous coating.
Currently, only a limited range of nanowire heights and array pitches can be produced in ordered arrays using metallic materials. Chemical vapor deposition, a method of nanowire fabrication with tunable height and pitch, produces metal nanowires in only disordered arrays (FIG. 1a) or carbon nanofibers having low electrical conductivity (FIG. 1b).
Electroplating metals in anodized aluminum oxide (AAO) template nanopores produces highly conductive and ordered arrays (FIG. 1c), but a maximum pitch of 2 μm typically causes the nanowires to cluster together due to local attractive forces.
These methods all utilize templates composed of dielectrics with vertical pores. Typically these templates are grown or deposited onto substrates, their pores are filled with metal, and they are dissolved away to produce freestanding wires on the substrate. A notable drawback of the present fabrication processes is that only certain dimensions can be accessed. If the aspect ratio of the wires becomes too large they are likely to cluster due to van der Waals' interactions, losing their freestanding nature. This clustering inhibits many material deposition processes from conformally coating the wires, limiting application of the nanowire array to core-shell and nanocoax architectures. Clustering also changes the geometry of the nanowire array, impacting applications such as plasmonic devices that would utilize the interaction of incident radiation with freestanding wires.
There remains unmet need for novel fabrication processes of greater tunability of nanowire height and pitch and for MNAs with enhanced optical and electronic properties for applications in energy conversion and storage, material deposition, sensing, communication, and computation.